Interparticle Potential and the Phase Behavior of Temperature-Sensitive Microgel Dispersions

Molecular-thermodynamic models assisted with experimental measurements are applied to correlate and predict the volume transition and structural ordering of poly(N-isopropylacrylamide) (PNIPAM) microgel particles dispersed in pure water. The effective pair potential between PNIPAM particles is represented by a Sutherland-like potential where the size and energy parameters are correlated with particle radius and the solution osmotic second virial coefficients attained from static and dynamic light scattering experiments. Using a first-order perturbation theory for the fluid phase and an extended cell model for the crystalline solid, the calculated phase diagram indicates that an aqueous dispersion of PNIPAM particles may freeze at both high and low temperatures. At low temperature, the freezing occurs at large particle volume fraction, similar to that in a hard-sphere system, while at high temperature, the freezing is driven by strong van der Waals attraction due to the increase in the Hamaker constant of the microgel particles when they collapse. The phase diagram of PNIPAM dispersions predicted from the molecular-thermodynamic models agrees favorably with experimental observations.